Existing and future packet-oriented communications networks, for example ATM (asynchronous transfer mode) communications networks, provide a variety of monitoring strategies to monitor variable and fixed and guaranteed transfer rates of ATM cells and ATM cells belonging to a framework or transfer framework. In particular, in the case of ATM communications networks, loss priorities are assigned to transmitting ATM cells and, based on the loss priorities and based on loss priorities decisions, are made concerning the retransfer of the respective ATM cells in a communication system. In this connection, a monitoring procedure is used to decide on the retransfer or rejection of an ATM cell to the respective communications system based on the assigned loss priorities. In this way, assigning loss priorities to individual cells establishes which ATM cells can be discarded in the event of overload within an ATM communication system without the loss of real time information that is relevant to the connection.
Also, the “Traffic Management 4.1” proposal of the 1999 ATM Forum defines various traffic classes and connection types. These include constant bit rate (CBR) connections, variable bit rate (VBR) connections, available bit rate (ABR) connections, unspecified bit rate (UBR) connections and guaranteed frame rate (GFR) connections.
The constant bit rate connection type is used for virtual links, for which a given constant transmission bandwidth must be provided for the time in which a virtual link exists.
The variable bit rate connection type is defined for virtual links with variable and/or changeable transmission rate requirements in the “Traffic Management 4.1” of the 1999 ATM Forum proposal.
The available bit rate connection type makes possible applications that do not have assigned a specific transmission bandwidth. The applications can use the currently possible transmission bandwidths within the ATM communication network, whereby the respective available bit rate connection must be assigned a maximum and a minimum transmission rate in each case and is not allowed to go above or fall below these limits.
The unspecified bit rate connection type is not assigned any fixed cell loss information or cell delay times of the respective virtual connection. Instead, the unspecified bit rate connection type represents a best effort service class comparable to the type of service class that in practice is provided for Internet applications.
The guaranteed frame rate connection type is provided to support delay-tolerant applications that are guaranteed a narrow transmission bandwidth and which can be granted additional transmission capacity freed up during data traffic. In a guaranteed frame rate connection type, a frame's information is packed into ATM cells and all ATM cells of a given frame are assigned the same loss priority or cell loss priority information using cell loss priority bits (CLP bits), i.e., the header of the ATM cells assumes the same value of the CLP bit used to control overload in ATM communications systems as for the ATM cells of a frame of a virtual guaranteed frame rate connection.
If, for example, a network node or an ATM communications system are overloaded, it could result in all of the ATM cells belonging to the network node or an ATM communications system being discarded. Experts in the field know of a number of different anti-overload strategies, such as frame discard, see the 1999 ATM Forum “Traffic Management 4.1” proposal. The above can avoid that additional ATM cells of the frame are transmitted via the provided transmission after loss or reception of a faulty ATM cell, although the frame information would not be received error-free at the end of the transmission line. The above would result in an unnecessary load on the ATM communications system. That is why in the event of an overloaded transmission line, it is especially important to remove the additional ATM cells of a frame as quickly and effectively as possible. If the ATM cells of a frame show different loss priorities, this frame will not support a quality of service, i.e., if an overload situation occurs some or all of the frame's ATM cells can be discarded in the network node or in the ATM communications system.
We also know ITU-T standards I.610 and I.371 provided solely for the operation, maintenance and administration and also resource management of ATM cells and control cells within the ATM communications system. Such ATM cells are called operation administration maintenance (OAM) cells and resource management cells, respectively. These can be inserted into the continuous ATM cell stream by ATM communications transmitters and an ATM communications system. When inserting OAM cells or RM cells into the cell stream, it is especially important to ensure the quality of service for the frames to be sent.